Device for stretch forming metal sheets, and having a sheet metal die



.luhr 3, 1956 R. A. LALL] 2,752,982

DEVICE FOR STRETCH FORMING METAL SHEETS, AND HAVING A SHEET METAL DIE Filed May 15. 1952 2 Sheets-sneer l INVENTOR pom efA ZQZZL' ATTORNEY July 3. 1956 R. A LALLI 2,752,932

DEVICE FOR STRETCH FORMING METAL SHEETS, AND HAVING A SHEET METAL DIE Filed May 15, 1952 2 Sheets-Sheet 2 24 l k n I I Hl l PIIM P qjn I 2 25- 1 g v a A 53 F d L m 52 Z2 INVENTOR JLL ll 20/7780 A. ZQZZL' BY I ATTORNEYS DEVICE FOR STRETCH FORMING lVlETAL SHEETS, AND HAVING A SHEET METAL DIE Romeo A. Lalli, Bridgeport, Conn. Application May 15, 1952, Serial No. 287,936

6 Claims. (Cl. 153-32) The present invention relates to a device for stretch forming metal sheets and to a sheet metal die and the method of making the same.

rates Patent Heretofore, in making thin sheet metal members such as are used as the covering material for an airplane and the like it has been the practice to hammer the sheets in order to produce the required contours therein. This is a slow and tedious operation.

in an eifort to speed up the operation dies have been provided for stretch forming the sheet metal. have been made of molded or cast steel or iron which results in very heavy structures which have been employed in large stretching presses requiring substantial amounts of floor space. These dies and presses have required many men to set them up and operate them as Well as substantial time and equipment.

Furthermore, the dies in their manufacture consumed much time and efiort since they were produced by forming a grid and making a paster cast thereon to the shape to be formed. The cast was then employed to form a mold for the die and the die is molded in solid steel or the like and prepared for mounting in the press. If these molds are to be used with magnesium sheets they must be bored out at the required location to receive the heating units which are in fixed relation therein.

The present invention overcomes the diificulties heretofore presented by producing a die from sheet metal which may be readily fabricated and, because of its light weight, easily handled in use whereby the time required and the cost of making and using the die is reduced. The die may be formed into a unitary stretching device which saves time and labor in operation and greatly reduces space requirements for housing the devices.

In carrying out the present invention a sheet metal die plate, preferably of steel is hammered to the desired contour as established by a sample or mock up of the piece to be formed. The die plate is then secured to a supporting box or frame positioned beneath the plate. I have discovered that a die thus formed is sufiiciently rigid for the stretch forming of magnesium or the like sheets. The die has the further advantage in that its hollow interior may readily house heating elements which may be adjustably positioned therein to provide part or all of the heat required to heat the magnesium sheets which have to be worked while hot.

By providing a stretching means on the die frame, an individual press may be formed therefrom which requires very little space and which requires no set-up time for changing dies as was occasioned when the dies were mounted in large presses.

The sheet metal die of the present invention may be also used in large presses if required for stretch forming aluminum and Duralumin or the like materials. When so used, it is preferred to fill the hollow space in the die with a high compression concrete which supports the undersurface of the die plate during the stretch forming operation in the press.

These ice By the present novel method of fabricating dies of sheet metal, it will be seen that the number of operations involved in making a die and the amount of material involved are both greatly reduced so that a substantial saving in time and cost are obtained.

Other features and advantages of the invention will be apparent from the specification and claims when considered in connection with the drawings in which:

Figure 1 shows a partially exploded perspective view of the stretch forming device.

Fig. 2 is a sectional view taken along line 2-2 of Fig. 3.

Fig. 3 is an end view.

Fig. 4 is a side view partially in section of another form of stretch forming device.

Fig. 5 is an end View of Fig. 4.

Fig. 6 is a longitudinal sectional view of a sheet metal die made in accordance with the present. invention and filled with high compression concrete.

. In Figs. 1 to 3, I have illustrated a sheet metal stretch forming device for use in shaping ductile sheet metal, i. e. magnesium sheets such as are used to form skins for airplanes and the like. The device comprises a die 10 having stretching means 11 thereon. The die, in accordance with the present invention, is a sheet metal die comprising a die plate 12 which is preferably formed from cold rolled steel or equivalent material. I prefer to use a sheet of cold rolled steel of about one-eighth of an inch thick which is shaped to the outline of the piece to be formed. It is then hammered on a hammering machine. to provide the required contours Which can be taken directly from a sample of the piece to be formed or a mock up of the same.

As shown in Figs. 1 and 3, the die plate has an outer contoured surface provided with an outwardly convex i curvature longitudinally and transversely thereof.

After the die plate has been formed a supporting frame 13 of sheet metal or the like is provided. This frame has side walls 14, 15 and end walls 16, 17 and a bottom wall 13 forming a hollow, box-like structure. The die plate has its edge portion only secured to the top of the box in any suitablemanner so as to support the plate in projecting position above the box frame to provide a die of suificient rigidity to withstand the stretching pressures. In the preferred form of the invention it is secured by spot welding since it has been found that by so securing the die plate the heat, during the securing operation, is kept to a minimum thus resisting any tendency to distort the die plate.

In making the die for use in stretch forming magnesium sheets, which sheets are most satisfactorily stretched when heated to temperatures of about 350 F,

I line the inner Walls and bottom of the box frame with heat insulating material 19 and provide heating elements 20 Within the hollow box under the die plate to heat the plate and thereby heat the sheet. In the illustrated form of the invention these heating elements are shown as electric heating elements Whose heat may be adjusted in any suitable manner and they are adjustably mounted in the box so as to be capable of movement thereon to extend under that part of the die plate having the greatest amount of curvature therein to which the sheet of magnesium must be formed.

In accordance with my present invention sufficient heat may be produced within the die to properly shape the sheet of magnesium. However, if additional heat is required exterior heaters such as heat lamps 21 shown in Fig. 2 may be set up so that heat therefrom will be directed to the outer surface of the magnesium sheet to heat the same.

The die may be mounted on any suitable support. As shown in Figs. 1 to 3, it is carried by a supporting frame '2 a 22 which can be readily fabricated from channel iron and angle iron and to which the die may be secured by welding or any other suitable means.

An important feature of the invention resides in the fact that these novel sheet metal dies can be converted, with slight. modification, into an individual stretching press. This is accomplished by providing adjacent one end of the die a fixed clamp 23 having jaws 23a, 23b moved by clamping bolts 230 to grip one end of the sheet as shown in Fig. 2. At the other end of the die is a movable clamp 24 mounted on the end of a lever 25 pivoted to the cross member 26 of the frame so that the clamp will move away from the end of the die in a substantially tangential manner. The clamp 24 has jaws 24a, 2412 moved by clamping bolts 24c to grip the opposite end of the sheet. Clamps 23 and 24 are shaped or curved to conform to the end curvature of the die. Due to the curving of the clamps they may be located adjacent the ends of the die and in this way substantial saving in material is produced and long gripping edges which ultimately become waste are avoided.

Power means are connected to the lever to provide a force thereto and hence to the movable clamp and the sheet gripped thereby to cause the magnesium sheet to be stretched over the die. In the herein illustrated form of the invention the force applying means comprises a hydraulic means embodying a piston and rod 27 pivoted to the movable lever 25 and a cylinder 28 pivotally connected at 28a to a frame member 22a of the frame 22. A source of fluid pressure which tends to maintain a uniform pull on the sheet may be applied to the cylinder to produce the controlled stretching action. While this may take any suitable form, in the herein illustrated form of the invention it comprises a hand pump 29 having a gauge 30 thereon calibrated to indicate the pull in pounds aplied to the sheet, thus providing for accurate control thereof whereby the flow of the metal being stretched can be controlled.

From the foregoing it will be seen that a relatively inexpensive press for stretch forming the sheet is provided which takes up a minimum amount of space in a plant. Further because each die is its own press, there are provided a plurality of presses each capable of producing the shaped pieces without the necessity of down time and the use of men and equipment to change dies as heretofore required when large stretching presses were employed.

In operation a sheet of magnesium 31 of the required size is mounted on the die by inserting one end between the clamping surfaces of the jaws 23a, 23b of fixed clamp 23 and the jaws moved to clamping relation by tightening the bolts 23c. The other end is similarly clamped to the jaws 24a, 24b of the movable clamp 24. The heaters are energized to heat the sheet of magnesium to approximately 350 P. which I have found to produce the desired flow of the metal. If desired a suitable lubricant may be applied to the die plate. The pump 29 is operated to actuate the piston and cylinder means and apply a pulling force to the clamp to draw the sheet into position over the die. I have found that with 20 x 68 x .025 inch magnesium sheets a pulling force of about 4000 pounds produces satisfactory stretching without tearing the material. As the heated piece stretches and relieves the pulling force, the pump is operated to maintain the pulling force substantially constant until the sheet is pulled completely into contact with the die plate and stretch formed to the contoured configuration of the die plate. The temperature and time specifications as set up by the U. S. Navy for forming sheet plate magnesium alloys H (Hard rolledmin. hardness after rolling, Rockwell M-101) used in aircraft skins is as follows:

Temperature-- Time, max. 275 max 6 350 max 15 400 max A After the stretching operation is completed, pressure in the cylinder is released relieving the force applied to the lever whereupon the lever returns by gravity to its original position and the formed sheet is freed from the die plate. The clamps are then released and the sheet removed.

in the form of the invention shown in Figs. 4 and 5, there is shown a double die for producing two parts simultaneously. The die is a sheet metal die and is made in a manner similar to that of Fig. l by hammering the die plate 12 to the required contour, securing it to the supporting box 13 mounted on the supporting frame fabricated of channel and angle iron. Heating elements 20 are mounted within the box to underlie that part of the die plate which includes the greatest amount of curvature. The two curved clamps 23, 24 are located adjacent each end of the die and extend for substantially the full width thereof. The clamps are carried by a transverse arm 32 to which hydraulic means in the form of a piston and rod 27 and cylinder 28 is connected. The transverse arm is normally urged into raised position by means of springs 33, which have one end connected to the arm and the other end connected'to the frame 22, and is moved downwardly by the piston and cylinder to apply a continuous substantially uniform pulling force on each end of the sheet 31 to cause the metal to flow and the sheet to stretch into the desired contour of the die plate. After the stretching operation is complete, the pressure in the cylinder is released and the springs return the transverse arm to raised position to release the sheet from the die plate whereupon the clamps are released and the piece is removed and split into its two parts.

The invention has been described thus far in connection with stretch forming magnesium sheets. I have found that the individual presses can also be used for making slight bends in aluminum and SO Duralumin which is worked cold. If the sheet metal dies are to be used to shape harder aluminum or Duralumin to provide substantial curved surfaces this is accomplished in a large stretching press in which case the sheet metal die as shown in Fig. 6 is employed. This die is made in the economical manner as heretofore explained by having the die plate 12 hammered to the required shape and boxed in with a support 13 which is mounted on a frame 22. The interior of the hollow die is then filled with a high compression concrete 34 which supports the die plate against the increased pulling forces required to shape the aluminum and Duralumin sheets, I

A feature of the present invention resides in the novel method of making the dies wherein it is merely necessary to hammer the die plate to conform with a sample of the piece to be made or to mock up or template thereof. The die plate can be readily secured to a fabricated supporting box. I prefer to spot weld the plate to the box adjacent the edge portion thereof so as to avoid any tendency to distort the plate should a continuous weld be used. Since the die plate and fabricated supporting box can be easily made, it will be readily apparent that substantial time may be saved in manufacturing dies over that heretofore employed when a cast had to be made, a mold made from the cast, and the die had to be molded out of solid steel or the like material. The same time saving method may be used in making hollow heated dies for magnesium or for filled dies for harder materials such as aluminum or Duralumin. Because of the method of making the dies, heaters may be easily installed within the hollow interior of the die as required without having to drill or otherwise modify the die as in the case of a solid die. Furthermore the heaters may be adjustably positioned as desired rather than having fixed locations as when solid dies were made.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. A device for stretch forming ductile metal sheets comprising a frame having a fixed hollow die secured thereto including a hammered sheet steel die plate provided with an outer face to receive and support the sheet to be formed during the stretching operation, said plate having an outwardly convex curvature longitudinally and transversely thereof conforming to the shape of the piece to be formed thereon, and a separate supporting box frame of sheet metal extending around the plate and secured to the undersurface of the edge portion thereof to support said plate only at the edge portions in projecting position above the box frame to provide a die of sufficient rigidity to withstand the stretching pressures; a pair of clamps at opposite ends of the die having gripping surfaces extending for substantially the full width of the sheet to grip the edges of the sheet being formed, at least one of the clamps being mounted on the frame for relative movement with respect to the die; means for applying a force to the movable clamp and sheet held thereby to cause said sheet to be drawn against said projecting hammered die plate and stretched over the outer contoured surface thereof so as to be shaped thereto; and means for applying heat to the sheet during the stretching thereof.

2. A device for stretch forming ductile metal sheets comprising a frame having a fixed hollow die secured thereto including a hammered sheet steel die plate provided with an outer face to receive and support the sheet to be formed during the stretching operation, said plate having an outwardly convex curvature longitudinally and transversely thereof conforming to the shape of the piece to be formed thereon, and a separate supporting box frame of sheet metal extending around the plate and secured to the undersurface of the edge portion thereof to support said plate only at the edge portions in projecting position above the box frame to provide a die of sufficient rigidity to Withstand the stretching pressures; a pair of clamps at opposite ends of the die having gripping surfaces extending for substantially the full Width of the sheet to grip the edges of the sheet being formed, at least one of the clamps being mounted on a lever pivoted to the frame for relative movement with respect to the frame; means for applying a force to the movable clamp and sheet held thereby including piston and cylinder means connected to the frame and lever to cause said sheet to be drawn against said projecting hammered die plate and stretched over the outer contoured surface thereof so as to be shaped thereto; and means for applying heat to the sheet during the stretching thereof.

3. A device for stretch forming ductile metal sheets comprising a frame having a fixed hollow die secured thereto including a hammered sheet steel die plate provided with an outer face to receive and support the sheet to be formed during the stretching operation, said plate having an outwardly convex curvature longitudinally and transversely thereof conforming to the shape of the piece to be formed thereon, and a separate supporting box frame of sheet metal extending around the plate and secured to the undersurface of the edge portion thereof to support said plate only at the edge portions in projecting position above the box frame to provide a die of suflicient rigidity to withstand the stretching pressures; a pair of clamps at opposite ends of the die having gripping surfaces extending for substantially the full Width of the sheet to grip the edges of the sheet being formed, at least one of the clamps being mounted on the frame for relative movement with respect to the die; means for applying a force to the movable clamp and sheet held thereby to cause said sheet to be drawn against said projecting hammered die plate and stretched over the outer contour surface thereof so as to be shaped thereto; and means for applying heat to the sheet during the stretching thereof comprising electrical heater units adjustably mounted within the hollow die whereby the heater units can be readily adjusted so as to be disposed under that portion of the sheet steel die plate having the greatest curvature so that heat from the heater will; be quickly and efiiciently transferred through the sheet steel die plate to the sheet being formed to cause the same to be more easily shaped.

4. A device for stretch forming ductile metal sheets comprising a frame having a fixed hollow die secured thereto including a hammered sheet steel die plate provided with an outer face to receive and support the sheet to be formed during the stretching operation, said plate having an outwardly convex curvature longitudinally and transversely thereof conforming to the shape of the piece to be formed thereon, and a separate supporting box frame of sheet metal extending around the plate and secured to the undersurface of the edge portion thereof to support said plate only at the edge portions in projecting position above the box frame to provide a die of sufficient rigidity to withstand the stretching pressures; a pair of clamps at opposite ends of the die having gripping surfaces extending for substantially the full width of the sheet to grip the edges of the sheet being formed, at least one of the clamps being mounted on the frame for relative movement with respect to the die; means for applying a force to the movable clamp and sheet held thereby to cause said sheet to be drawn against said projecting hammered die plate and stretched over the outer contoured surface thereof so as to be shaped thereto; and means for applying heat to the sheet during the stretching thereof comprising inner and outer heater units, the inner units consisting of electrical heater units adjustably mounted within the hollow die whereby the heater units can be readily adjusted so as to be disposed in that portion of the sheet steel die having the greatest curvature and the outer heater units comprising heat lamps spaced from the outer surface of the die plate to heat the outer surface of the sheet being formed whereby the sheet being formed will be heated from heat transferred quickly and efiiciently through the sheet metal die plate and heat directly transferred from the lamps to cause the sheet being formed to be more easily shaped.

5. A device for-stretch forming ductile: metal sheets comprising a frame having a fixed hollow die secured thereto comprising a hammered sheet steel die plate provided with an outer face to receive and support the sheet to be formed during the stretching operation, said plate having an outwardly convex curvature longitudinally and transversely thereof conforming to the shape of the piece to be formed thereon, and a separate supporting box frame of sheet metal extending around the plate and secured to the undersurface of the edge portion thereof to support said plate only at its edge portions in projecting position above the box frame to provide a die of sufiicient rigidity to withstand the stretching pressures; a pair of clamps at opposite ends of the die having gripping surfaces extending for substantially the full Width of the sheet to grip the adjacent edges of the sheet being formed, said clamps being pivotally mounted on the ends of a transverse arm disposed below the hollow die; spring means between the arm and frame normally urging the arm toward the die; piston and cylinder means connecting the arm and frame for applying a force to the arm to move the arm and clamps away from the die to cause said sheet to be drawn against said hammered die plate and stretched over the outer contoured surface thereof so as to be shaped thereto; and means for applying heat to the sheet during the stretching thereof.

6. The invention as defined in claim 1 wherein the means for applying heat to the sheet comprises heaters disposed within the box frame, and wherein the box frame has a bottom closure plate to confine the heat therein.

References Cited in the file of this patent UNITED STATES PATENTS 20,128 Pitcher Apr. 27, 1858 (Other references on following page) 7 v UNITED STATES PATENTS Filstrup Dec. 19, 1899 Jeter July 20, 1915 Haskell Feb. 17, 1920 Lockwood Aug. 31, 1920 Claybourn Mar. 25, 1924 Jolstad Mar. 29, 1927 Kellogg Apr. 23, 1929 Ridder July 12, 1932 Ragsdale Nov. 21, 1933 Ford Apr. 27, 1943 Bergstrom Nov. 30, 1943 Rechton Mar. 27, 1945 Leary June 12, 1945 8 Lermont June 19, 1945 Seligman Dec. 4, 1945 Ahearn Nov. 12, 1946 Walsh June 29, 1948 Maize July 19, 1949 Hercik Nov. 1, 1949 Johnson Nov. 21, 1950 Briscoe Mar. 20, 1951 OTHER REFERENCES Pages 378 to 382 of the Welding Handbook, third edition, published by American Welding Society in 1950. 

1. A DEVICE FOR STRETCH FORMING DUCTILE METAL SHEETS COMPRISING A FRAME HAVING A FIXED HOLLOW DIE SECURED THERETO INCLUDING A HAMMERED SHEET STEEL DIE PLATE PROVIDED WITH AN OUTER FACE TO RECEIVE AND SUPPORT THE SHEET TO BE FORMED DURING THE STRETCHING OPERATION, SAID PLATE HAVING AN OUTWARDLY CONVEX CURVATURE LONGITUDINALLY AND TRANVERSELY THEREOF CONFORMING TOTHE SHAPE OF THE PIECE TO BE FORMED THERON, AND A SEPARATE SUPPORTING BOX FRAME OF SHEET METAL EXTENDING AROUND THE PLATE AND SECURED TO THE UNDERSURFACE OF THE EDGE PORTION THEREOF TO SUPPORT SAID PLATE ONLY AT THE EDGE PORTIONS IN PROJECTING POSITION ABOVE THE BOX FRAME TO PROVIDE A DIE OF SUFFICIENT RIGIDITY TO WITHSTAND THE STRETCHING PRESSURES; A PAIR OF CLAMPS AT OPPOSITE ENDS OF THE DIE HAVING GRIPPING SURFACES EXTENDING FOR SUBSTANTIALLY THE FULL WIDTH OF THE SHEET TO GRIP THE EDGES OF THE SHEET BEING FORMED, AT LEAST ONE OF THE CLAMPS BEING MOUNTED ON THE FRAME FOR RELATIVE MOVEMENT WITH RESPECT TO THE DIE; MEANS FOR APPLYING A FORCE TO THE MOVABLE CLAMP AND SHEET HELD THEREBY TO CAUSE SAID SHEET TO BE DRAWN AGAINST SAID PROJECTING HAMMERED DIE PLATE AND STRETCHED OVE THE OUTER CONTOURED SURFACE THEREOF SO AS TO BE SHAPED THERETO; AND MEANS FOR APPLYING HEAT TO THE SHEET DURING THE STRETCHING THEREOF. 